The invention relates to an improved coupling arrangement for a device for measuring the angular displacement of two objects rotating relative to each other, such as in a machine.
Devices for measuring the angular displacement of an object relative to another object are known in the art. Typically, such a device includes a rotatable shaft to which is attached a graduated plate. The rotatable shaft is secured by some means to a first rotating object. The angular displacement of the graduated plate is measured by a scanning unit which typically includes a photosensor which counts or scans the graduated plate as it passes through the scanning unit. Usually, the scanning unit and graduated plate are encased in a housing which is attached to a second object.
One such prior art device includes a rotatable shaft which is attached to a rotating object to be measured by a flexible coupling which is as torsionally rigid as possible. The housing is secured or clamped to a second object. Certain problems are presented with this type of measuring device. One problem is that the flexible coupling requires additional space outside the housing. Moreover, especially when a hollow rotatable shaft is used, it is difficult to construct a coupling between the rotatable shaft and the first rotating object that will transmit the angular displacement of the rotating object with the high degree of accuracy needed. Also, due to angular accelerations of the first rotating object, torsional vibrations are created which have an adverse effect on the position regulating circuit of a numerical control device for the machine.
Another prior art device for measuring the angular displacement of an object relative to another object utilizes a shaft which is rigidly attached to a rotating object. In this device, the housing is secured to the second object by a follower or articulated link which is as torsionally rigid as possible. A problem with this design is that due to the eccentricities of the shaft and of the shaft connection with the spindle, combined with the distance of the follower or the articulated link from the shaft axis angular errors which are not tolerable for precise measurements can be created. Moreover, forces acting on the housing from outside the device are conducted over the shaft bearing onto the shaft, and thereby effect the running precision of the bearing as well as the accuracy of the angular displacement measurement.
German Pat. No. 29 06 432 describes a coupling system for an apparatus for measuring the angular displacement of two objects relative to each other in which the shaft is rigidly secured to a first rotating object and the housing is coupled to a second stationary object over a parallel guide. In this type of coupling system there is a problem that forces from outside the apparatus are conducted onto the rotatable shaft and can result in an inaccurate measurement. These external forces can damage the coupling rods and their bending springs. Moreover, the coupling system utilized in this device requires an inordinate amount of space.
Thus, there is a need for a device for measuring the angular displacement of two objects relative to each other which overcomes the problems of the prior art.